This invention relates generally to internal combustion engines, and more particularly to an improved compression release mechanism for single cylinder, four stroke engines.
Compression release mechanisms are well known in the art. Generally, means are provided to hold one of the valves in the combustion chamber of the cylinder head slightly open during the compression stroke while cranking the engine. This action partially relieves the force of compression in the cylinder during starting, so that starting torque requirements of the engine are greatly reduced. When the engine starts and reaches running speeds, the compression release mechanism is rendered inoperable so that the engine may achieve full performance. It is normally advantageous for the compression release mechanism to be associated with the exhaust valve so that the normal flow of the fuel/air mixture into the chamber through the intake valve, and the elimination of spent gases through the exhaust valve is not interrupted, and the normal direction of flow through the chamber is not reversed.
Examples of compression release mechanisms for four-stroke engines are shown in U.S. Pat. Nos. 3,381,676; 3,496,922; 3,897,768; and 4,977,868, all assigned to the assignee of the present application.
U.S. Pat. No. 4,977,868, the disclosure of which is hereby incorporated by reference, discloses a compression release mechanism for an internal combustion engine wherein a rotatable compression release pin is positioned axially parallel to the camshaft and rotatably received in the cams disposed on the cam shaft. The pin has an auxiliary cam surface mounted at an axial end thereof to extend beyond the outboard cam to engage one of the valve lifters at low engine speed. The other cylindrically-shaped axial end of the pin is press fit into a matching cylindrical bore in a cylindrical hub which extends perpendicularly from the flyweight. Undesirably, this arrangement could result in the compression release pin coming loose from the hub, in which event the auxiliary cam surface becomes misaligned. Further, in production, alignment of the pin within the bore in the hub is critical, but precise alignment is difficult to achieve.
An improved compression release mechanism that overcomes the above difficulties is desirable. Accordingly, it is desired to provide a compression release mechanism that is effective in operation and relatively simple in construction, and that may be utilized to actuate the exhaust valve in an internal combustion engine.
The present invention provides a mechanical compression release including a compression release pin that is non-cylindrically shaped at one axial end thereof. The non-cylindrically shaped end is received and secured into a correspondingly shaped bore disposed in a cylindrical hub extending perpendicularly from a one-piece flyweight. This configuration avoids the slipping problem described above, and the pin is much easier to align during assembly.
In one form thereof, the present invention provides a compression release mechanism for relieving compression during engine starting in an internal combustion engine having a camshaft rotatably disposed within a housing, the camshaft having inboard and outboard cams and a cam gear disposed thereon. The mechanism comprises a flyweight having a hub extending substantially perpendicularly therefrom. A non-cylindrically shaped bore is disposed in the hub. A release pin that has a first end having a shape corresponding to the bore is received in the bore. The release pin has a lift member at a second axial end thereof, which is adapted to selectively engage a valve lifter.
In a preferred form thereof, the corresponding shape is a D-shape, and the flyweight is integrally formed in one piece. Further, the release pin is also integrally formed in one piece. Optionally, an adhesive can be applied to the bore within the hub and the adhesive is activated when the first end of the pin is inserted into the bore.
According to another optional form, the end of the release pin which is inserted into the bore of the hub includes a small flat portion or a groove into which the hub is compressed or deformed by crimping so as to secure the release pin in the bore of the hub. In this embodiment, the end of the release pin that is received into the bore can be cylindrical or non-cylindrical.
In yet another optional form, the release pin includes a groove that is configured to receive a retaining ring or clip therein. In this later form, a retaining ring or clip can be used to secure the release pin in position.
An advantage of the present invention is that it provides an effective compression release mechanism that is operable to significantly reduce the cranking effort required to start an internal combustion engine without thereby sacrificing engine power and engine running speeds.
Another advantage of the present invention is that the non-cylindrically shaped bore and correspondingly shaped axial end of the release pin fit securely together so that the release pin does not become misaligned after a period of use.
Yet another advantage of the present invention is that it is much easier to assemble than prior art designs. The corresponding shapes of the non-cylindrical bore and axial end of the release pin ensure that the release pin can only be installed into the hub of the flyweight in a properly aligned position. Time-consuming alignment procedures are therefore unnecessary with the present invention.
A further advantage of the above invention is that it provides a compression release mechanism which is economical in construction and highly reliable in operation.
Another advantage of the present invention is that the flyweight is formed in one piece from nicad-zinc and will not rust.